DESCRIPTION (adapted from the Abstract): The goal is to enhance our understanding of the role the hippocampus plays in memory using a naturalistic model, memory for cache sites in food-storing birds, in which certain memory features have become modified during evolution in response to the selective advantage of retrieving stored food caches. This model system is unique in that specific experiences related to memorization have a dramatic impact on volume and neuron number of a distinct region of the brain, the avian hippocampal formation (HF), at a relatively late stage in development, after the young have left the nest. This result raises several questions about the types and amounts of experience needed to trigger these morphological changes and the time course over which they occur. Previous work has focused exclusively on HF in juveniles. However, HF has major connections with other telencephalic regions including archistriatum (ARCHI) and lobus parolfactorius (LPO). Therefore, the determination of whether or not neuronal changes can be detected in HF alone or additionally in other regions, and of what role these regions might play in memory is now important. Also, whether brain changes are restricted to juveniles or whether experience-dependent growth and atrophy occurs in the adult brain must now be tested. Further, because the intensity of food-storing fluctuates seasonally, changes in photoperiod may produce similar effects on brain plasticity. The intent of this research plan is to compare and contrast developmental and seasonal changes in food-storing behavior, memory, and the brain. Experiments will be conducted on food-storing mountain chickadees and black-capped chickadees. Bush tits will serve as a non-storing species control. The aims are: (1) to analyze the effects of food-storing experience and season on brain morphology in adults and juveniles by manipulating photoperiod and the opportunity to store; (2) to examine the nature of experiences needed to trigger growth and attrition by manipulating the amount of seeds stored and duration over which caches are stored; (3) to determine the time course over which changes occur by sampling juveniles with and without food-storing experience at different ages; (4) to assess the relative importance of programmed cell death and cell birth on brain morphology; and (5) to lesion HF, ARCHI,and LPO to assess the role each region plays in memory for cache sites, whether they are the same regions that show morphological change, and whether the effects of lesions are permanent.